The World's Simplest C# WPF MVVM Example as described here.
A C# 9 (.NET 5.0) version of the code can be found in the C#9.0 branch. 
A C# 8 (.NET Core 3.0) version of the code can be found in the c#8.0 branch. 
A C# 6 version of the code can be found in the C#6.0 branch. 
A C# 4 version of the code can be found in the C#4.0 branch. 
A C# 4 version of the code (with detailed comments by Hampton) can be found in the C#4.0 branch (with detailed comments). 